A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known scatterometers include angle-resolved scatterometers of the type described in United States patent application publication nos. US 2006-033921 and US 2010-201963. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in U.S. patent application publication no. US 2006-066855. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables measurement of overlay and other parameters on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a substrate. The intensities from the environment product structures can efficiently be separated from the intensities from the overlay target with the dark-field detection in the image-plane.
Examples of dark field imaging metrology can be found in U.S. patent application publications nos. US 2010-0328655 and US 2011-069292, which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in U.S. patent application publication nos. US 2011-0027704, US 2011-0043791, US 2011-102753, US 2012-0044470, US 2012-0123581, US2012-0242970, US 2013-0258310, US 2013-0271740 and in PCT patent application publication no. WO 2013-178422. Typically in these methods it is desired to measure asymmetry as a property of the target. Targets can be designed so that measurement of asymmetry can be used to obtain measurement of various performance parameters such as overlay, focus or dose. Asymmetry of the target is measured by detecting differences in intensity between opposite portions of the diffraction spectrum using the scatterometer. For example, the intensities of +1 and −1 diffraction orders may be compared, to obtain a measure of asymmetry.
In some of these prior patent application publications, it is proposed to perform dark-field metrology using different illumination modes and/or different image detection modes to obtain the +1 and −1 diffraction orders from periodic structures (gratings) within the target. On the other hand, such methods are susceptible to asymmetry in the optical paths used in the different modes, which will result in errors when measuring the asymmetry of the target. Accordingly, although various calibrations and corrections can be applied to reduce these errors, it is generally the case that best overlay measurement results are obtained if the target is measured twice under identical conditions of illumination and detection. To do this, for example, the substrate is rotated 180 degrees between measurements, to obtain the −1 and the +1 diffraction order intensities in turn. This mode of asymmetry measurement may therefore be referred to as a substrate rotation mode. The use of exactly the same optical path for both measurements ensures that any difference between the measured intensities is due to target properties, not properties of the scatterometer.